Salt in our Fresh Waters
Examining the increasing levels of salt in freshwater bodies across North America and how this issue might be addressed.
ABSTRACT:
There has been an observed increase in the amount of salt in freshwater bodies across the United States, Canada, Europe and elsewhere. This metric is found by measuring the amount of chloride per liter of water, as chloride is both a major component of salt and is easy to test for in a laboratory setting. The research that exists has found that not only has there been an increase of chloride over the last two hundred years, but also that the rate at which this increase occurs is quickening. While the increased levels are generally not harmful at present, these trends do justify action on the part of local, state and federal governments as well as their equivalents outside of the United States in order to preserve the integrity and usability of these bodies of water.
The research additionally examines potential causes of increased salinity, predictors of future salinity increase, and possible solutions to this problem. The overall findings seem to be that even a modest reduction in road salt usage in some of the largest contributing communities could be enough to slow the rate of salinization and allow the lakes to return to their historic levels of chloride per liter of water.
There’s a beautiful region of North America that’s peppered with freshwater lakes that ranges from way up north in Hudson Bay, Canada down to the southernmost point of Michigan and beyond; from the forests of Minnesota out east towards the coastal regions of Maine and New York. Thanks to the likes of the Mississippi, Ohio, and other such rivers throughout the continent, this hoard of water is shared far beyond their footprint. This bounty of freshwater are the remnants of the last ice age and glacial recession, and have been enjoyed by humans across the continent for countless generations. The reality of the lakes being fresh has been such a trivial fact of life for so long that few even consider a future where this isn’t the case. Unfortunately, given recent trends, that reality becomes more likely year after year.
As previously stated, these lakes are largely the byproduct of the last ice age when glaciers dominated the northern half of the continent. As they came and went, these massive walls of ice gouged the Earth leaving craters that would later become the lakes we know today. Some are small, measuring in at just a few square acres in total size, and others are so staggeringly large that they’re referred to as inland seas. In terms of size, shape, biodiversity, resident ecosystems and more they’re all incredibly varied. But, they do share an important and worrying similarity: all across the region, the relative salinity in these lakes is on the rise.
Millions of years ago the Great Lakes region was covered by an ancient ocean. As that ocean gradually subsided it left behind massive deposits of salt that lie dormant below much of the region to this day. In fact, there are mines currently operating in Detroit, Michigan that are harvesting these ancient deposits. By and large, the existence of subterranean salt doesn’t pose much of an issue, and in fact is reasonably ignored or even unknown to the majority of those who live above it.
These salt deposits don’t lie completely inert, however. Many of these lakes are deep, and at their lowest points can mingle with the salt, slowly creating a briny solution that’s diluted throughout the lake. This additionally happens with subterranean watersheds, which likewise brings more of this salt to aboveground lakes and rivers. Furthermore, there is some amount of atmospheric salt that is brought down to earth during periods of precipitation adding to the salt content. However, these processes have been going on for thousands of years and have largely reached an equilibrium across the various bodies of water. At least in the Great Lakes the historic level of chloride measured was approximately 1 milligram per liter of water. Chloride is generally used in studies as it’s a fairly accurate proxy for salt in general, and it tends not to degrade easily.
Were this natural equilibrium still in place, there’d be no reason to be alarmed. It’s intense and expensive work to desalinate water and the ecosystems and societies that interact with these lakes have evolved under these conditions. The amount of salt winding up in freshwater only becomes a problem once it starts accumulating and rising above this natural level.
Using records of chloride levels in the Great Lakes that date back over 200 years to the early 1800’s, researchers have found that this number has been slowly rising. What has historically been 1 milligram of chloride per liter of water is now approximately 15 times that amount. Furthermore the rate that this level is rising has been accelerating in recent decades, with a majority of that increase occurring since the 1970’s. Through a variety of studies it’s known that bodies of water across the United States, Canada and even in Europe are increasingly becoming salty. There’s also been a great deal of research into why that’s happening.
Starting in the 20th century it became common practice in the United States and elsewhere to use salt to help minimize the impact of ice on public roadways. While it has been incredibly effective for transportation and general safety, the rising salinity in freshwater environments has been a major unintended consequence. Estimates put the increase in freshwater salinity attributable to road salt at 50% or greater. There are other ways that humans can introduce salt into freshwater, such as industrial runoff or ground wells being dug too deep into their aquifer and extracting salt from underground deposits, but clearly these aren’t the main issues to be concerned with.
Road salt is such a prevalent issue that two accurate predictors of increased lake salinization are proximity of roads to the lakeshore and the density of roads relative to permeable land. Research shows that if even ≥ 1% of land within 500m of lakeshore is covered in roads, it’s likely that salinization will increase. This applies to thousands of lakes across the United States and Canada, and so should be taken seriously.
The impermeable surface created by roads can increase salinization in several ways. Perhaps the most obvious cause is that roads create a need for salting in the first place. Furthermore, since asphalt and other common materials used in road construction don’t allow for permeation, the saltwater created from melting ice rushes downhill and collects in bodies of water. This additionally increases the impact that precipitation plays in the salinization of freshwater, as less of the atmospheric salt is being held in the ground. All of these factors have contributed to rising levels of chloride in fresh water across the continent. In Lake Michigan specifically, this level has gone from about 1 milligram per liter of water to roughly 15 milligrams.
To be clear, this is still a far cry from the salinity levels of our oceans which are closer to 35 grams of chloride per liter of water. But even the increase seen thus far is cause for alarm. Given the rate that these chloride levels have been rising, in conjunction with the fact that this rate has been accelerating in recent years, it may be only a few decades until this freshwater isn’t so fresh. It only takes roughly 250 milligrams per liter for water to take on a distinctly salty taste and becomes unhealthy to drink, and researchers say that if current trends persist it’s possible that threshold is met within five decades. Well before that threshold, however, it begins causing damage to freshwater ecosystems.
Thankfully, in some ways this is already a moot point. As previously stated, local governments have been aware of the fact that chloride levels in freshwater bodies have been on the rise. They understand the risks that this poses to local ecosystems, residential and commercial use, as well as the expense of trying to tackle this problem as it becomes more severe. As such, research is being conducted on how to reduce the amount of salt that’s being added unnaturally through means such as road salting and other methods of polluting. Remember: there’s a natural level of salt that will always be present in these lakes and streams. Between geological and atmospheric conditions, the number will never be 0. A little salt isn’t a bad thing, but as the relative level creeps up it can cause numerous issues. Furthermore, the rate that humans have been increasing the chloride level in freshwater isn’t unmanageable. Once again, research shows that roughly half of the extra salt being dumped into these reservoirs comes from road salt usage. In the case of Lake Michigan specifically, it’s been found that roughly 70% of the salt being added comes from just 5 of its 300 tributaries. If the communities responsible for adding these excessive levels of salt to these tributary rivers could reduce their usage by even a modest amount, it could be enough to get the lakes back to a manageable level.
Anyone who lives somewhere that experiences winter temperatures well below freezing knows how dangerous the roads become with a little ice buildup, especially when not properly maintained. The answer isn’t to use no salt, but look into alternatives to supplement salt usage and to find more efficient application of the salt we do use. Michigan State University has been at the forefront of this issue, both in terms of researching the rising chloride levels and coming up with solutions on how we might address it. Currently across their campuses they use a salty brine instead of traditional rock salt, which both reduces the amount of salt used and applies it more evenly. By using this method they’re helping to reduce the amount of salt that runs off into the Grand and Red Cedar rivers, and thus into Lake Michigan.
The case of rising levels of salt in our freshwater is a testament to environmental monitoring and protection. Fresh water is critical to the survival of life on Earth, and civilizations rise and fall depending on their access to it. At this point governments are aware of the problem and are beginning to take action to nip it in the bud. It’s quite possible that several decades from now it has become a nonissue that fades into the background, never causing concern for the average citizen. However, if it goes unaddressed the threat remains. It’s important that we have researchers and government agencies putting their time and effort towards protecting that which we hold most dear and remains so critical to our wellbeing as a species and society.
